56 



sufficiently delicate to allow tLe necessary diftusion to 

 proceed ; such plaees, for example, are the surface of the 

 tentacles, of the inner lips, etc. 



Excretory Organs. 



Two kidneys are present, of which the right is much 

 the larger. 



The much-reduced Left Kidney (see figs. 4, 5, 8, 31} 

 is a small compact sac with only a limited excretory activity. 

 The wall of this kidney is thin where it abuts on the rectum. 

 l)ut is much thickened where it adjoins the pericardium. It 

 possesses renal epithelium like that of the right kidney, and 

 the amount of excretory matter visil)le is fairly large. The 

 concensus of opinion is in favour of its possessing a reno- 

 pericardial pore, a conclusion which our preparations 

 confirm. The thickened wall between the left kidney and 

 the pericardium is a heterogeneous mass made up of a 

 few muscle fibres and connective tissue, with occasional 

 granular cells of lymphatic function. In this mass are 

 numerous cavities which are blood lacunre. 



The Rigid Kidney, which seems to be the main 

 excretory organ throughout the Grastropod series, has, in 

 Pdtellei, been flattened out and spread around the visceral 

 hump, in correlation and connection with the great 

 perivisceral blood sinus. It is a large structure made 

 up of several lobes (figs. 4, o, 8, ol) : — 



(a) An anterior dorsal lobe, extending superficially over 

 the front of the visceral hump behind the pericardium 

 and rectum. 



(b) A posterior or perivisceral lobe, extending around 

 the visceral hump backwards along the right side, and 

 then a fair distance forwards along the left. In very old 

 limpets (a) and (h) are united distally. 



(c) A ventral lobe, extending on the ventral surface 



